To fix this, romcnv_mvs.exe is included in http://nj-emu.tfact.net/files/njemu_source_20071028_231dev.zip. Unfortunally all the windows builds of romcnv_mvs.exe that iv’e tried have not managed to generate the crom cache (you get a 0 sized crom file). And the result for most games that require cache will be that the game boots, but most graphics is omitted.

So this is how to fix it. You will need linux (or probably any unix flavor) and gcc / make.

extract http://nj-emu.tfact.net/files/njemu_source_20071028_231dev.zip.
cd njemu/romcnv
mv makefile.mvs Makefile
UNIX=1 make

This will build romcnv_mvs for you.

Then run : ./romcnv_mvs /path/to/roms/that/need/cache -all
This will create a directory named cache in your current directory (njemu/romcnv/ if you havnt changed it)
copy the content of cache/ to ms0:/PSP/GAME//cache/
and your good to go.